1. Field of the Invention
The present invention relates to a lighting apparatus such as that used in an LCD (Liquid Crystal Display), and more particularly to a lighting apparatus comprising serially-driven lighting units.
2. Prior Art of the Invention
The improved quality and reduced costs of modern LCDs are making the LCD an increasingly popular choice in the field of display devices. LCDs are traditionally employed in notebook computers and other portable computer systems. LCD devices have made great progress in conjunction with the evolution of computer displays from the conventional VGA (Video Graphics Array) standard to the newer XGA (Extended Graphics Array) standard. Nowadays, LCD devices have a superior display quality to that of CRTs (Cathode Ray Tubes), and are poised to replace conventional CRT devices.
The LCD cannot achieve light-emission independently; it generally has to rely on a backlight source. The backlight source and related elements are indispensable in a typical direct-viewing type of LCD device. The performance of the backlight source significantly influences the display quality of the LCD device. Moreover, the backlight source is a large contributor to the cost and power consumption of the LCD device.
A typical backlight source of an LCD is implemented by utilizing several lamps. Apparatuses and means for driving these lamps are important technologies. FIG. 4 illustrates a prior art lighting apparatus 8, which includes N lamps 10 and 2N transformers 12, 14. The 2N transformers 12, 14 are connected to the N lamps 10 so that both ends of each lamp 10 are respectively connected to two corresponding of the transformers 12, 14. Taking the initial lamp 10 as an example, one end of the lamp 10 is connected to the initial transformer 12, while the other end of the lamp 10 is connected to a second transformer 14. Generally speaking, the lamp 10 is driven by a high voltage, and this high voltage is even higher than the voltage of the power supply. Therefore, when the voltage of the power supply is applied to the lamp 10, the voltage has to be boosted to a higher level. The transformers 12, 14 connected to both ends of the lamp 10 are used to boost the voltage. Because each lamp 10 needs two transformers 12, 14, the N lamps 10 need the 2N transformers 12, 14.
The lighting apparatus 8 commonly employs numerous lamps 10 in order to illuminate a large LCD, thus requiring a large number of transformers 12, 14 to be used in the system. Backlight modules for this kind of display system are unduly large, and result in increased production costs.
FIG. 5 illustrates another prior art lighting apparatus 18. The lighting apparatus 18 includes N lamps 20 and N transformers 22. As shown, each lamp 20 is controlled by an individual transformer 22. A positive high-voltage end and a negative high-voltage end of the lamp 20 are connected to the transformer 22, so that the boosted high voltage provided by the transformer 22 is applied to the lamp 20. The N lamps 20 only need N transformers 22, which significantly reduces the above-described disadvantages of the lighting apparatus 8.
In the lighting apparatus 18, a feedback control circuit to control the current of the N lamps 20 should be employed, so that the lighting apparatus 18 is more stable. A feedback control mechanism achieves two main purposes. Firstly, the brightness of each lamp 20 needs to be adjusted by adjusting the current of the lamp 20. The feedback control mechanism can finely increase or decrease the current of the lamp 20 according to feedback signals in order that the lamp 20 has a proper level of brightness. Secondly, the feedback control mechanism makes the current stable. If distortions in the current occur without any feedback, the brightness of the lamp 20 becomes unstable. However, in the lighting apparatus 18, both high voltage ports of the corresponding transformer 22 are connected to the single lamp 20, which means that the lighting apparatus 18 cannot be provided with the feedback control mechanism directly. Therefore, in practice, the lighting apparatus 18 is not provided with a feedback control mechanism, and does not have the advantages of being able to make the lighting system stable and being able to accurately adjust the brightness of the lamps 20. This is contrast with the lighting apparatus 8, in which the transformers 12, 14 each have one high voltage port connected to ground. That is, the lighting apparatus 8 can be provided with a feedback control mechanism directly.
In summary, in the lighting apparatus 8, the numerous transformers 12, 14 increase overall size and production costs. In the lighting apparatus 18, a feedback control mechanism cannot be provided, which makes it difficult to control the lamps 20. With ongoing improvements in LCD technologies, there is increasing demand to enhance both the cost-effectiveness and the quality of LCDs. However, conventional lighting apparatuses are unable to adequately meet this demand.